1. Field of the Invention
The present invention relates to a power supply apparatus, and more particularly to a power supply apparatus which uses a battery as a power source and is suitable as a power supply for a portable electronic apparatus such as a portable computer.
2. Description of the Related Art
In a power supply apparatus having a DC-DC converter for obtaining a regulated power supply of a desired voltage from an unregulated DC power supply, a diode is inserted in series with a power supply line to prevent current from flowing back from the output side. In an apparatus that permits connection of an AC adapter, as well as a battery, as an input to the DC-DC converter, a diode is inserted to prevent current from flowing into the battery from the AC adapter.
The diode inserted in series with the power supply line exhibits a given forward voltage drop V.sub.f. Therefore, to make effective use of the battery and to ensure supply of the desired voltage despite a decrease in battery voltage, it is desirable that the diode forward voltage drop be reduced as much as possible. As a diode having a relatively small forward voltage drop V.sub.f, an expensive Schottky barrier diode is known. Even this type of diode exhibits a forward voltage drop ranging from 0.3 to 1 V, and it is desirable that the voltage drop be further reduced, particularly for a power supply apparatus using a battery as a power source.